1. Field of the Invention
The present invention relates to an alignment mark detecting optical system for detecting a state of misalignment between two objects to be aligned, more particularly an optical system for detecting the relative position between the two objects, such as a mask and a wafer, which are conjugate with respect to a projection lens, in an exposure and alignment apparatus for manufacturing semiconductor circuits.
2. Description of the Prior Art
The recent trends of higher operation speed and larger capacity of semiconductors constantly force to reduce the minimum projectable width of a line in a circuit pattern. As is well known, the semiconductor circuit elements are manufactured by superposing plural complicated patterns on a semiconductor substrate of Si or the like. One of the problems with the further miniaturization is how fine patterns can be projected on the substrate, and another problem is how accurate is the alignment between the patterns superposed. Generally, the accuracy of the alignment is required to be not more than 1/5-1/10 of the minimum line width of the circuit patterns. Therefore, it is not rare that the lack of accuracy of the alignment is fatal in the manufacture. The recent appearance of an automatic alignment mechanism which performs the alignment discussed above, automatically, is within this background. The automation alignment device has released the operators from simple and routine work and brought about the high speed, uniform quality and high precision alignment operation.
The major point in achieving a desirable automatic alignment device is how to detect the information to be processed. The electric processing of signals is not so flexible as in the case where an operator directly observes the objects and brings them into alignment. This is why various expedients are required in an observation optical system or an exposure optical system to provide better signal-to-noise ratio.
The mask, one of the objects to be aligned, has a relatively simple structure having a glass plate and a patterned thin layer of chromium or chrominum-chromium oxide. The wafer, however, has a complicated structure resulting from the plural superposed patterns and subsequent impurity diffusion and the other processing, so that the state of its surface widely varies. In addition, the wafer has on its surface a thin layer of photoresist whenever it is exposed to the mask pattern, and the thickness of the photoresist and the unevenness of its thickness influence the detection of the wafer signal.
In order to obtain a good signal-to-noise ratio in sensing the wafer which has the above described variable nature, many proposals have been made. One of such proposals is in Japanese Laid-open Patent Application 132851/1977 by the assignee or applicant of the present application. In this prior art, the object is scanned by a spot or slit of a laser beam which can provide a high brightness, and in order to increase the signal-to-noise ratio, only the scattered beam is picked up as the light to be directed to a photosensor. If this were applied, as it is, to an alignment scope wherein the wafer is observed through the mash, a problem would arise as to the interference of the laser beam. Particularly, the light directly scattered by the mask and the light returned, after the scattering, by way of the wafer, interfere with each other. The resultant signal is not constant with respect to time, which leads to unstable and unreliable measurement.
In order to avoid the occurrence of the undesirable interference, a proposal has been made in Japanese Laid-open Patent Application No. 24504/ 1981, wherein a projection optical system is disposed between the mask and the wafer, the projection optical system having therewithin an element corresponding to 1/4 .lambda. plate (a quarter wave plate) to separate the light directly reflected by the mask from the other light with the use of polarization. It is a serious limitation in the design of the projection optical system that an element corresponding to a .lambda./4 plate must be inserted therein. For those reasons, the conventional apparatus uses an interchangeable lens which has a .lambda./4 plate and which replaces a part of the projection lens at the time of alignment operation. As an alternative, a mirror in a projection optical system is provided with a special coating.